The present invention relates to a semiconductor storage apparatus, and more particularly to a read only memory (hereinafter called as a "ROM") having a copy guard function.
FIG. 10 shows a structure of a conventional ROM. The ROM is composed of an input buffer circuit 1 to which an address signal is supplied from outside, an address buffer circuit 1 to which an output signal from the input buffer circuit 2 is supplied, an address decoder 3 to which an output signal from the address buffer circuit 2 is supplied through an address bus, a memory cell 4, a sense amplifier 5 to which data in the memory cell 4 is supplied, and an output buffer circuit 11 to which an output signal from the sense amplifier 5 is supplied. The address signal supplied from the outside is stored in the address buffer 2 through the input buffer circuit 1. The address decoder 3 selects a memory cell at an address corresponding to the address signal stored in the address buffer 2. Data stored in the selected memory cell is supplied to the output buffer circuit 11 through the sense amplifier 5, and then output from the output buffer circuit 11 to the outside.
A ROM having the above-mentioned structure permits the contents thereof to easily be copied by reading data therein by sequentially making accesses to the addresses in the ROM to cause another nonvolatile memory or the like to sequentially store the data. If a ROM, in which software of, for example, a video game has been stored, is copied, the manufacturer and the dealer sustains a great economic loss. Therefore, copy guard of data in the ROM is an important issue.